Portable hot plates are often used for warming and maintaining the temperature of heated beverages, such as coffee, tea, hot apple cider, etc., and food such as soups. These devices are also used in the laboratory for heating substances. Hot plates generally include a supporting surface upon which the base of a container (e.g., a coffee cup, chemistry flask, etc.) is placed while undergoing heat transfer from a heat source within the hot plate. An electric resistance heating element is typically used for the heat source and dissipates enough energy to achieve a moderately high to high temperature (e.g., up to several hundred degrees C.).
A major drawback of most portable hot plates is that they represent a significant risk of burning anyone who contacts the surface that supports a container to be heated when the surface is hot. Children are especially at risk. In order to maintain a beverage such as coffee at a desired temperature, it is necessary that the heat transferred from the hot plate to the container match the heat lost from the container (and the liquid contained therein) to the environment.
Since it is desirable to keep heated beverages hot, coffee cups and the like are typically made of materials that have a relatively low thermal conductivity, such as refractory materials, glass, plastic, etc. However, the insulating properties of these materials act as an impediment in heating a beverage in the container with a hot plate. As a result, the hot plate must be maintained at a temperature much higher than the desired temperature of the beverage. This temperature is often high enough to cause moderate to severe burns to anyone who accidentally touches the hot plate when it is at its normal operating temperature.
The manner in which hot plates are typically used exacerbates the foregoing problem. Most portable hot plates have a power switch, and may also include a temperature control. In order to heat something with a hot plate, it is necessary to at least turn the power switch on and it may be necessary to set the temperature control knob to a desired setting. It is not uncommon to accidentally leave a hot plate energized after it has been used. Furthermore, even if properly turned off after use, the power switch can easily be turned on by a curious child, posing a significant safety hazard for children, as well as a potential fire hazard.
It is therefore desirable to provide a portable apparatus for heating a substance, such as a beverage or food, that does not require the use of a high temperature heat source in the apparatus. Furthermore, it is desirable that such an apparatus only operate when the substance is being heated and automatically shut off when a container containing the substance is removed from the apparatus.